The term calaminarian grassland is used for the distinctive plant communities associated with heavy metal mining, particularly the sparse vegetation of lead mine spoil and smelt mill waste where only metal-tolerant plants (known as metallophytes) can grow. The lead and other heavy metals are toxic to plants, the soils are nutrient poor, and the ground is often very dry in summer. The only plants that can grow here are small and slow-growing, and specially adapted to these stressful conditions.
The most easily recognised form of calaminarian grassland in the Yorkshire Dales is a short, open turf with exposed stones and areas of bare ground close to historic lead mining sites. The most common plants are sheep’s fescue and spring sandwort, which was used by miners as an indicator of a lead vein beneath the surface. Thyme, ribwort plantain, and thrift often grow with them. The bare ground between them supports a rich variety of lichens and mosses, and these are an important part of the plant community. Some of the lichens, such as Sarcosagium campestre and species of Vezdaea, are tiny and can only be seen with a hand lens, but they are quite beautiful and well worth the effort! There may be a living crust of cyanobacteria and other micro-organisms over the soil surface.
More grassy areas, where bent grass and harebell are more common, may have populations of alpine penny-cress. This plant is only found on heavy-metal contaminated sites in Britain, and accumulates astonishing levels of zinc in the shoot which deters grazing animals. Mountain pansy does well on less-contaminated ground.
Wetter ground also has some distinctive species, including Pyrenean scurvy-grass and the large green lichen Peltigera leucophlebia.
According to the Botanical Society of Britain and Ireland: “This grass is of enormous agricultural and socio-economic importance, being one of the predominant constituents of hill pastures across great swathes of upland Britain and Ireland”.
This is the main grass of ‘Calaminarian grasslands’ and varieties of this species (so-called ‘eco-types’) have become adapted to soils with a high level of heavy metal content that would be lethal to most other grasses.
Spring sandwort has a delicate appearance that can look somewhat out of place against the rugged environment of the old lead-mine spoil heaps or limestone scars where it is found in northern England. Also known as ‘Vernal sandwort’ or ‘Leadwort’, it is a small tufted perennial with white flowers and can grow to form small pockets or larger carpets of plants.
In the Yorkshire Dales National Park it is fairly frequent on calaminarian grassland which establishes on lead mining spoil. Good examples include Thoralby lead mines and Ballowfield Local Nature Reserve in Wensleydale and near Bastow Wood in Wharfedale. It can also be seen on some short limestone grasslands, rock ledges and limestone scars in the south west of the National Park.
If you’ve ever seen an old twelve-sided threepenny bit, you might recognise the thrift as it is the emblem displayed on the back of the coin. The thrift family name ‘Pumbaginaceae’ derives from the ancient belief that these plants could cure lead poisoning. And it is now known that thrift is a metallophyte which means it can tolerate soils rich in heavy metals such as lead. The plant was once also used to treat obesity.
With its pretty pink clusters of flowers, thrift is sometimes known as ‘sea-pink’, ‘cliff clover’ and ‘ladies’ cushions’.
Thrift is not common within the Yorkshire Dales National Park but can be seen at Ballowfield Local Nature Reserve in Wensleydale and Kilnsey Moor in Wharfedale where it grows near the old lead mine spoil heaps.
In former lead mining areas, the soil is often rich in heavy metals and not all plants are able to grow in these conditions. However, the Alpine pennycress is part of a group of plants called metallophytes, which can tolerate or actively accumulate heavy metals like lead, nickel or zinc from the soil.
Delicate in appearance, the alpine penny-cress has white or purple flowers with violet anthers and heart-shaped seed pods.
Its distribution in the British Isles is very localised and is found mainly on the Pennine hills.
In the Yorkshire Dales National Park, alpine pennycress can be seen in flower between April and August on bare and sparsely grassed stony places in old lead mining areas. It may also be found on limestone near Malham in Malhamdale, in upper Wharfedale and at Ballowfield Local Nature Reserve in Wensleydale.
Scurvygrass is so-called because the leaves have a high vitamin C content and before the availability of citrus fruit, scurvygrass was widely used by sailors to prevent scurvy.
Although it’s more likely that the common scurvygrass was used for this purpose, due to its widespread coastal distribution, Pyrenean scurvygrass is thought to have similar properties.
This sprawling plant has short stems and small white flowers which develop into egg-shaped seed pods.
Pyrenean scurvygrass is a metallophyte which means it is a plant that can tolerate high levels of heavy metals such as lead. As a result it can be seen in flower between April and August close to historic lead mining sites in the Yorkshire Dales National Park. It also grows beside streams and rivers throughout the National Park.